Try This one :-
protected void Button1_Click(object sender, EventArgs e)
{
Con = new SqlConnection("Your Connection String");
Con.Open();
Cmd = new SqlCommand("update employee set
Emp_Name='"+TextBox1.Text+"' where Emp_ID=5",Con);
if (Cmd.ExecuteNonQuery() > 0)
{
Con.Close();
Response.Write("Done..");
}
}